Projection objective and projection exposure apparatus with negative back focus of the entry pupil

ABSTRACT

The disclosure concerns a projection objective, which can include an object plane in which an object field is formed, an entry pupil, a mirrored entry pupil (RE) in a mirrored entry pupil plane obtained by mirroring the entry pupil (VE) at the object plane, an image plane, an optical axis, at least a first mirror and a second mirror. The projection objective can have a negative back focus of the entry pupil, and a principal ray originating from a central point of the object field and traversing the objective from the object plane to the image plane can intersect the optical axis in at least one point of intersection, wherein the geometric locations of all points of intersection lie between the image plane and the mirrored entry pupil plane.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit under U.S.C. §119 to U.S. provisionalapplication No. 60/786,744, filed Mar. 27, 2006. This application alsoclaims priority under U.S.C. §119 to German Patent Application No. 102006 014 380, filed on Mar. 27, 2006. The full disclosure of theseearlier applications is incorporated herein by reference.

FIELD

The disclosure relates to a projection objective and a projectionexposure apparatus.

BACKGROUND

An illumination system has been presented in US 2005/0088760, whereinthe rays reflected from a reflective object in an object plane enter theprojection objective on divergent paths. In the case of an axiallysymmetric projection objective which has an optical axis, this meansthat the projection objective has a negative back focus of the entrypupil. For axially symmetric systems, this means that a positiveprincipal ray angle γ is present at the reflective object in the objectplane. In the examples presented in US 2005/0088760, the positiveprincipal ray angle γ is less than 7° (e.g., less than 6°).

In the case of a negative back focus of the entry pupil a mirrored entrypupil in a mirrored entry pupil plane is obtained by a mirror reflectionon the object plane. The mirrored entry pupil plane lies in this case tothe image side of the object plane.

A positive back focus is present if the principal ray angle γ in theobject plane is negative, i.e. if the principal ray of the central fieldpoint after the reflection on a reflective object in the object plane,for example on the reflective reticle, proceeds on a path that convergestowards the optical axis. In the case of a positive back focus of theentry pupil, the entry pupil of the projection objective lies to theimage side of the object plane and, accordingly, the mirrored entrypupil plane lies to the opposite side of the object plane.

The back focus is determined by the distance from the object plane tothe point where the principal ray directed to the central field point ofthe illuminated field in the object plane intersects the optical axis.With a positive principal ray angle at the object or at the reticle, forexample γ=8°, and with a field radius r=125 mm, the back focus S_(EP) isobtained as S_(EP)=−R/tan γ=−889.4 mm. In systems with a negative backfocus of the entry pupil, the principal ray angle γ at the object ispositive.

SUMMARY

According to a first aspect of the disclosure, a first embodiment of amicrolithography projection objective with a negative back focus of theentry pupil is proposed, which includes at least two mirrors, i.e. afirst mirror (S1) and a second mirror (S2), wherein the objective isdesigned in such a way that each principal ray CR originating from acentral point of the object field and traversing the objective from theobject plane to the image plane intersects the optical axis (HA) atleast once in a point of intersection that is specific to that ray, withthe respective points of intersection being located geometricallybetween the image plane of the projection objective and the mirroredentry pupil plane of the mirrored entry pupil of the projectionobjective. The terms “entry pupil” and “mirrored entry pupil” areexplained in more detail in FIG. 1 a.

According to the first aspect of the disclosure, all points ofintersection of the principal rays with the optical axis of theprojection objective lie between the mirrored entry pupil plane and theimage plane of the projection objective.

In an advantageous embodiment, the mirrors can be arranged withrotational symmetry relative to the optical axis HA.

In an advantageous embodiment of the disclosure, the at least one pointof intersection has along the optical axis a first distance A1 to theobject plane, and the mirrored entry pupil has a second distance A2 tothe object plane, wherein the distances A1 and A2 conform to the rulethat A2 is always smaller than A1 (e.g., A2<0.9·A1, A2<0.8·A1,A2<0.7·A1, A2<0.5·A1).

An objective of this kind leaves enough design space in the area of themirrored entry pupil to allow for example an optical element to bearranged there.

In this application a projection objective is a projection systemarranged to image radiation from an object plane to an image plane withthe aid of a plurality of optical elements arranged to direct radiationfrom the object plane to the image plane.

In particular, if a projection objective of this kind with a negativeback focus of the entry pupil is used in a projection exposureapparatus, it opens the possibility for a modular design. Projectionexposure apparatus consist in general of an illumination system and aprojection objective. The illumination system serves to illuminate afield in an object plane, and the projection objective serves to projectan image of an object that is arranged in the object plane into an imageplane. In projection exposure apparatus of modular configuration, thegeometry of the ray paths is chosen so that the elements of theillumination system are arranged in a first design space and theelements of the projection objective in a second design space. Thus, theillumination system forms a first module and the projection objectiveforms a second module. Each of the modules can be separated from theapparatus without thereby affecting the other module. For example, ifthe illumination system requires servicing or adjustment, one can forexample exchange the illumination system without the projection systembeing affected by this exchange.

Furthermore, a projection exposure apparatus of this kind isdistinguished by a higher transmittance, because with a negative backfocus of the entry pupil it is possible to save mirrors in theillumination system, as is described in US 2005/008760.

In systems according to the disclosure, an optical element such as forexample a pupil facet mirror of a double-facetted illumination systemcan be arranged with a negative back focus in the area of the opticalaxis of the projection objective, since this design space is not neededby the mirrors of the projection objective and also since no rays of theimaging ray pattern pass through the design space taken up by a facetmirror of this kind.

In some embodiments, the projection system is configured so as to avoida grazing-incidence mirror which would otherwise be arranged in theillumination system before the object plane for the purpose of foldingthe ray paths. This measure likewise serves to increase thetransmittance of a microlithography projection exposure apparatus inwhich an objective of this kind is being used.

In certain embodiments of the disclosure, the projection objectiveincludes at least four mirrors (e.g., at least six mirrors).

In some embodiments of the disclosure, only one aperture stop plane isformed in the projection objective. The objective can be divided into afirst and a second sub-objective with a first and a second number ofmirrors, wherein the second sub-objective includes the aperture stopplane. The second sub-objective can have two mirrors, i.e. the fifth andthe sixth mirror in an embodiment that has a total of six mirrors, whilethe first sub-objective comprises the first, second, third and fourthmirrors.

The image-side numerical aperture of the projection objective withnegative back focus of the entry pupil according to the disclosure canmeet the condition NA≧0.2 (e.g., NA≧0.25, NA≧0.3).

In one design of the projection objective, the mirror surface of thefirst mirror is configured as a convex mirror surface, the mirrorsurface of the second mirror is configured as a concave mirror surface,the mirror surface of the third mirror is configured as a concave mirrorsurface, the mirror surface of the fourth mirror is configured as aconvex mirror surface, the mirror surface of the fifth mirror isconfigured as a convex mirror surface, and the mirror surface of thesixth mirror is configured as a concave mirror surface.

In order to provide the longest possible drift paths inside theobjective, it is envisioned that an embodiment of the projectionobjective which has at least six mirrors or exactly six mirrors issubdivided into a first partial objective with a first and a secondmirror, and a second partial objective with a third, fourth, fifth andsixth mirror, wherein the geometrical distance between the first partialobjective and the second partial objective along the optical axis islarger than 30% of the overall length of the objective (e.g., largerthan 40% of the overall length of the objective, larger than 50% of theoverall length of the objective, larger than 60% of the overall lengthof the objective). The term “overall length of the objective” means thedistance from the object plane of the projection objective to the imageplane as measured along the optical axis.

In some embodiments of the disclosure it is envisioned that in aprojection objective with negative back focus of the entry pupil, anincident principal ray on its way to the first mirror (CRE) of theprojection objective travels in the meridional plane of the projectionobjective on a path between the principal ray reflected from the mirrorsurface (CRR) and the optical axis (HA) of the projection objective.This is shown in FIG. 1 h. Both the incident principal ray (CRE) as wellas the reflected principal ray (CRR) are in this case associated withthe same field point, for example the central field point.

With the principal ray traveling from the object plane to its incidenceon the first mirror (CRE) on a path that runs in the meridional plane ofthe projection objective between the principal ray reflected from themirror surface (CRR) and the optical axis (HA) of the projectionobjective, one achieves the benefit that enough design space isavailable particularly in the anterior part of the objective for theinstallation of an optical element, for example the pupil facet mirrorof the illumination system or a grazing-incidence mirror.

In an alternative embodiment of the projection objective it can beenvisioned that at least one intermediate image may be formed in thelight path from the object plane to the image plane.

In an alternative embodiment of a projection objective with negativeback focus of the entry pupil, the objective is designed so that in ameridional plane of the projection objective the light path from theobject plane to the first mirror (S1) crosses the light path from thesecond mirror (S2) to the image plane and that no further mirror isplaced between the object plane and the first mirror (S1).

The term “meridional plane” in the present context means the plane thatcontains the optical axis (HA) of the projection objective and thecentral field point of the field in the object plane. Advantageously,the light path in the projection objective crosses over itself in thepart of the objective that lies closest to the object plane. This partof the projection objective comprises those mirrors which have a shortdistance to the object plane in which the reticle is arranged. The lightbundle which proceeds from the object to the first mirror can cross thelight bundle which travels from the second to the third mirror. Thismakes it possible that in particular the second mirror can be arrangedat a large distance from the optical axis. The aperture stop can bearranged between the second and the third mirror and has a large axialdistance from the object plane, as does the third mirror.

A projection objective of this kind can contain four mirrors. In someembodiments, a projection objective of this type can contain sixmirrors.

In the projection objectives according to the disclosure with a negativeback focus of the entry pupil, the large distance of the first andsecond mirrors from the optical axis makes it possible to arrange anoptical element, specifically an optical integrator, at or close to thepoint of intersection of the optical axis with the principal ray (CR)that is directed to the central field point.

In contrast to the systems with negative back focus that have beendisclosed in US 2005/0088760, the embodiments of projection objectivesaccording to the disclosure no longer require a mirror to be arrangedbefore the object plane for the purpose of folding the light path. Thetransmittance of the system is thereby significantly increased.Furthermore, by using a projection objective of this kind in aprojection exposure apparatus, one avoids a cross-over between the lightpaths in the illumination system and in the projection system, so thatthe projection exposure apparatus can have a modular design with regardto its spatial arrangement and technical construction.

In some embodiments of a projection objective that has an intermediateimage, the latter can be formed between the fourth and the fifth mirrorof the objective, if the objective contains six mirrors.

In certain embodiments where the projection objective with negative backfocus in accordance with the disclosure is configured as an objectivethat contains six mirrors, the mirror surface of the first mirror isconcave-shaped, the mirror surface of the second mirror isconcave-shaped, the mirror surface of the third mirror is convex-shaped,the mirror surface of the fourth mirror is concave-shaped, the mirrorsurface of the fifth mirror is convex-shaped, and the mirror surface ofthe sixth mirror is concave-shaped.

The aperture stop can be arranged between the second and the thirdmirror. The image-side numerical aperture NA of the projection objectivecan be more than 0.2 (e.g., more than 0.25, more than 0.3).

In some embodiments, the objective according to the disclosure is acatoptric projection objective with a negative back focus of the entrypupil, an image-side wave front aberration W_(RMS) of less than 0.01λand on each of the mirrors a maximum angle of incidence smaller than21°. The image-side wave front aberration can be W_(RMS)≦0.07λ (e.g.,W_(RMS)≦0.06λ). The maximum angle of incidence in the meridional planeon each of the mirrors can be ≦20°. The symbol λ stands here for thewavelength of the light which traverses the projection objective alongan imaging light path from the object plane to the image plane.

The projection objective in certain embodiments has at least fourmirrors, wherein the first and the fourth mirror in a light path fromthe object plane to the image plane are convex mirrors.

The maximum mirror diameter of all mirrors in the meridional plane canbe <190 mm (e.g., <180 mm).

All of the afore-named individual measures such as for example the sizeof the aperture, the number of mirrors of an objective, angle ofincidence, mirror diameter etc. can be arbitrarily combined for all ofthe embodiments shown here without thereby deviating from the subject ofthe disclosure. All of these combinations are within the scope of whatis being disclosed about the disclosure.

In addition to the microlithography projection objective the disclosurealso provides a microlithography projection exposure apparatus. Themicrolithography projection exposure apparatus according to thedisclosure includes an illumination system that is traversed by anillumination light bundle, and it further includes an object plane inwhich an object field is illuminated, and a projection objective,wherein the latter is traversed by an imaging light path from the objectplane to the image plane and wherein the projection objective has anentry pupil with negative back focus. The microlithography projectionexposure apparatus is designed so that the illumination light bundlewhich travels to the object plane from an optical component of theillumination system that is in the next-to-last position in the lightpath does not cross the imaging light path in a meridional plane exceptin the vicinity of the reflective object, i.e. at the reticle.

In a projection exposure apparatus with negative back focus of the entrypupil, it is possible to arrange an optical element, for example partsof the optical integrator, or for example the second facetted opticalelement—the so-called pupil facet mirror, in the area of the mirroredentry pupil. As a result, the optical imaging element for producing animage of the second facetted optical element can be omitted.

This makes it possible to specify microlithography projection exposureapparatus which in accordance with the disclosure are designed so thatthe transmittance is increased in comparison to known projectionexposure apparatus of the state of the art with positive back focus. Theterm “transmittance” means that portion of the light which is emitted bythe light source, passes through the microlithography projectionexposure apparatus, and falls on the image plane in which the object tobe projected is arranged. The number of mirrors in a microlithographyprojection exposure apparatus is of particular significance for thetransmittance, because the reflectance of the multi-layer mirrors at EUVwavelengths is at most around 70%. If a microlithography projectionexposure apparatus of a first design contains two mirrors more than amicrolithography projection exposure apparatus of a second design, thetransmittance in the microlithography projection exposure apparatus ofthe first design is reduced by the two additional mirrors approximatelyby a factor of 2 in comparison to the second design as the reflectanceof each of the two mirrors is about 70%.

Since the respective paths of the illumination light rays and of theprojection light rays are no longer crossing each other, a modulardesign becomes possible for the overall design of the system accordingto the disclosure, meaning that the illumination system can be separatedfrom the projection objective with regard to its spatial arrangement andtechnical construction. The modular design is achieved by arranging theelements of the illumination system in a first design space and theelements of the projection objective in a second design space. Inmicrolithography projection apparatus of this kind, there are verysimple ways of separating the illumination system from the projectionobjective.

In a first embodiment of the disclosure, the layout of the projectionobjective is designed in such a way that the projection objective has nointermediate image in the light path from the object plane to the imageplane. In an alternative embodiment, the objective has a cross-over ofthe ray paths in that part of the objective that is closest to theobject plane, i.e. has a shorter geometrical distance to the objectplane.

The illumination system of a microlithography projection exposureapparatus which comprises an objective with a negative back focus of theentry pupil can be configured in different ways.

Thus, a first embodiment can be designed in which the illuminationsystem is a double-facetted illumination system.

A double-facetted illumination system is distinguished by having a firstfacetted mirror with a multitude of first facets, so-called fieldfacets, as well as a second facetted mirror with a multitude of secondfacets, so-called pupil facets. As described above, a system of thiskind has the second facetted element with pupil facets arranged in ornear the mirrored entry pupil of the projection objective. The pupilfacet mirror in some embodiments can have about 200 to 300 pupil facetswhich, as an option, can be designed so that they are switch-controlledwhereby the correlation of the first facets to the second facets can bechanged. A change in the correlation of the first to the second facetsfor the adjustment of the setting can be achieved in a double-facettedillumination system for example by exchanging the first facetted opticalelements with field facets. In systems that are designed for awavelength ≦193 nm (e.g., for wavelengths ≦100 nm, for wavelengths inthe range of EUV wavelengths of 10 to 30 nm), the facets are designed asreflectors, i.e., mirrors.

Instead of a double-facetted illumination system, it is also possible tospecify an illumination system where a diffuser is arranged in or nearthe mirrored entry pupil of the projection objective. A diffuser of thiskind can for example have a multitude of diffusion centers. Thesediffusion centers can for example be constituted by 500 to 1000 or moresmall mirror facets which are arranged on a carrier, or the diffusioncenters can be formed by a holographic lattice. Light that falls on thisdiffuser is scattered in all directions by the diffusion centers.

Due to the arrangement of the diffusion centers—primarily in a round orslightly oval form—a light source is produced which scatters theincoming light from the light source in the predetermined shape, namelyin an arcuate shape, into large solid-angle elements.

This kind of a design with a diffuser plate has the advantage that, incontrast to a double-facetted illumination system, one fewer opticalelement needs to be used for the illumination of the field in the objectplane and for the illumination of a pupil plane, whereby thetransmittance is increased in comparison to the double-facettedillumination system.

To control the setting of the illumination in a pupil plane, the designmay include that an aperture stop is placed in the light path before orafter the diffuser.

A particularly advantageous embodiment of an illumination system mayinclude a location-variant or field dependent C or optical integratorfor the illumination of the field plane as well as of a pupil plane or aconjugate pupil plane. A diffuser of this kind is also referred to as aspecular reflector. Since the specular reflector does not need to bearranged in the pupil plane or in a conjugate plane associated with thepupil plane, the microlithography projection exposure apparatus can bedesigned in such a way that the diffuser is optimally placed in relationto other system components in the light path in the light path. Thediffuser can be of a size that makes it simple to produce the individualfacets with a high accuracy of their angles. The size of the mirrorfacet can be larger than 2 mm (e.g., larger than 3 mm, larger than 5mm).

For an optimal, i.e. largely loss-free, illumination of the diffuserwhich can be designed in a kidney shape, it is intended to place anoptical component before the diffuser in the light path from the lightsource to the diffuser, more specifically a normal incidence mirrorwhich can be configured as a free-form surface with an off-axis conicalcomponent. A mirror of this kind is a part of a cone which does notinclude the cone axis. When an extra-axial segment of a cone of thiskind is illuminated, one obtains a not quite complete annularillumination which largely matches the kidney shape of the diffuser.

Placing a normal-incidence mirror ahead in the light path has thefurther advantage of providing a filtering effect. The multi-layeredcoating of the normal-incidence mirror reflects in essence only theradiation in the usable part of the wavelength spectrum and thus has theresult that only useful light arrives at the diffuser, i.e., light inthe wavelength range of for example λ=13.5 for EUV systems. Thefiltering effect of the multi-layered mirror is independent of the shapeof the mirror.

If the individual facets of the diffusers are to be configured as planarmirrors, a design that offers advantages in the manufacturing process,it is possible to arrange an optical element, more specifically anormal-incidence mirror in the light path after the diffuser. With thisarrangement, the normal-incidence mirror that is placed behind thediffuser, projects a strongly magnified image of the light source intothe object plane that contains the object which is to be illuminated.The location-variant or field dependent diffuser produces in this objectplane a multitude of images of the source which are superimposed on eachother. If the normal-incidence mirror is in addition designed to haverefractive power, it is possible to set the scale ratio of the image bymeans of the normal-incidence mirror. As a result, one always obtainsthe same illumination in a pupil plane independent of the size of thelocation-variant or field dependent diffuser, based on the set imagingscale ratio.

This makes it possible to design the diffuser in a very large size,which has the advantage that due to the large surface the radiationexposure of the diffuser is reduced and the thermal stress on thediffuser is thereby reduced.

Instead of the afore-described mirror with a free-form surface with anoff-axis conical component, the kidney-shaped illumination of thelocation-variant or field dependent diffuser can also be obtained bygiving a kidney-shaped design to the collector that collects the lightof the light source and reflects it onto the diffuser. Due to the smallnumber of reflections, this concept provides a particularly efficientillumination system with a high transmittance. In some embodiments, agrazing-incidence collector can be used as collector of themicrolithography projection exposure apparatus, and the ray pattern inthe illumination system does not include an intermediate image. Thisallows the diffuser to be arranged directly before the reticle.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples of embodiments of the disclosure are hereinafter described andillustrated in drawings, wherein

FIG. 1 a serves to visualize the negative back focus of the entry pupil,

FIG. 1 b represents a schematic view of a microlithography projectionexposure apparatus,

FIG. 1 c represents a cone of rays for the definition of the image-sidenumerical aperture,

FIG. 1 d represents a part of a mirror surface in a meridional section,

FIG. 1 e represents a part of a mirror surface in a plane that isorthogonal to the meridional plane,

FIG. 1 f shows the shape of a ring field,

FIG. 1 g shows a microlithography projection exposure apparatusaccording to the state of the art as described in US 2005/088760 with aprojection objective with negative back focus of the entry pupil,

FIG. 1 h is a schematic illustration to explain the geometry of the raypath in the vicinity of the first mirror,

FIG. 2 a represents a first example that embodies a projection objectiveaccording to the disclosure with a negative back focus of the entrypupil, wherein no intermediate image is formed in the light path fromthe object plane to the image plane,

FIG. 2 b consists of Table 1,

FIG. 3 a shows an alternative version of a projection objectiveaccording to the first embodiment,

FIG. 3 b consists of Table 2,

FIG. 4 shows a microlithography projection exposure apparatus thatcontains a projection objective according to FIG. 2 a,

FIG. 5 a shows a second embodiment of a projection objective withnegative back focus of the entry pupil, wherein the paths of the lightray cross over themselves in the meridional plane in the first partialobjective,

FIG. 5 b consists of Table 3,

FIG. 6 a represents a first alternative system of a projection objectiveaccording to the second embodiment,

FIG. 6 b consists of Table 4,

FIG. 6 c shows a second alternative system of a projection objectiveaccording to the second embodiment,

FIG. 6 d consists of Table 5,

FIG. 7 represents a first embodiment of a microlithography projectionexposure apparatus with a projection objective with negative back focus,a normal-incidence mirror that serves as a narrow-band wavelengthfilter, as well as a first and a second facetted optical element,

FIG. 8 represents a microlithography projection exposure apparatus witha diffuser that is arranged in the pupil plane or in the light pathafter the pupil plane,

FIG. 9 shows a third embodiment of a microlithography projectionexposure apparatus,

FIG. 10 shows a fourth embodiment of a microlithography projectionexposure apparatus with a location-variant or field dependent diffuser,

FIG. 11 shows the light-ray pattern from the light source to thediffuser, wherein the location-variant or field dependent diffuser ispreceded in the light path by a normal-incidence mirror with a free-formsurface and a conical component,

FIG. 12 represents a fifth embodiment of a microlithography projectionexposure apparatus with a location-variant or field dependent diffuserand a collector mirror that is arranged in the light path before thediffuser for the kidney-shaped illumination of the diffuser, and

FIG. 13 represents a sixth embodiment of a microlithography projectionexposure apparatus with an illumination system without an intermediateimage and with a location—variant or field dependent diffuser.

The disclosure will now be described by referring to the drawings whichrepresent examples without implying any limitations.

DETAILED DESCRIPTION

FIG. 1 a to 1 h will be referred to in the following detaileddescription of the general concepts which are used in all of theembodiments and relate to all of the illustrated examples.

FIG. 1 a serves to visualize the concept which is referred to herein asnegative back focus.

FIG. 1 a shows the principal ray CRB of an illumination light bundledirected at the central field point of an illuminated field asillustrated for example in FIG. 1 f. As shown here, the principal rayCRB of the illumination light bundle reflected on a reflective objectREFLOBJ, for example a reticle and, now as principal ray of a projectionlight bundle, enters into a projection objective of which the firstmirror S1 and the second mirror S2 are shown in the drawing. “Negativeback focus of the entry pupil” means that the principal ray angle γ atthe reflective object, for example the reticle, is positive. Theprincipal ray angle γ is the angle by which the principal ray CRP isinclined relative to the normal direction NO of the reflective objectREFLOBJ. For systems with negative back focus of the entry pupil, theangle γ is by definition positive and is measured counterclockwise.

The mirrored entry pupil RE is obtained by mirroring the entry pupil VEat the object plane in which the reflective object REFLOBJ is arranged.With a negative back focus of the entry pupil VE, the mirrored entrypupil RE is formed to the image side of the object plane. In otherwords: the mirrored entry pupil RE and the image plane, in which e.g.the wafer is situated, lie on the same side with respect to the objectplane.

FIG. 1 b represents a microlithography projection exposure apparatus2100. The latter contains a light source 2110, an illumination system2120, a projection objective 2101 as well as a support structure or worksurface 2130. Further shown is a Cartesian coordinate system. Theradiation of the light source 2110 is directed to an illumination system2120. The illumination system 2120 has an influence on the radiationoriginating from the light source 2110, for example by homogenizing theradiation or by directing a ray bundle 2122 of the radiation for exampleby means of the illustrated direction-changing mirror 2121 onto a mask2140 which is positioned in an object plane 2103. The projectionobjective 2101 projects the radiation that is reflected by the mask 2140onto a substrate surface 2150 which is positioned in an image plane2102. The ray bundle 2142 on the object side has according to thedisclosure a principal ray CRP with a positive principal ray angle γ.Also indicated in the drawing is the surface-normal direction NO of theobject plane 2103 in the vicinity of the mask 2140. The substrate 2150is supported or carried by a support structure 2130, wherein the supportstructure 2130 moves the substrate 2150 relative to the projectionobjective 2101, so that the projection objective 2101 projects images ofthe mask 2140 onto different areas of the substrate 2150.

The projection objective 2101 includes an optical axis 2105. As shown inFIG. 1 a, the projection objective 2101 projects a part of the mask 2140that does not contain the optical axis of the projection objective 2101into an image plane 2102. The light source 2110 is selected in such away that it provides electromagnetic radiation at an operatingwavelength λ that is used to operate the microlithography projectionexposure apparatus. In some of the examples described, the lights source2110 is a laser plasma source or a plasma discharge source that emitsEUV radiation. As alternatives, it is also possible to use light sourcesthat are used for other wavelengths, such as for example light-emittingdiodes (LEDs) which emit radiation in the blue or UV range of theelectromagnetic spectrum, for example at 365 nm or 248 nm, respectively.This can be for systems in which broad-band light sources are usedtogether with mirror systems.

The operating wavelength λ of the microlithography projection exposureapparatus lies in the ultraviolet or extreme ultraviolet (EUV) range ofthe electromagnetic spectrum. The operating wavelength can for examplebe 193 nm or less, especially 100 nm or less. In the examples ofembodiments described herein, the operating wavelength can for examplebe in the range of 193 nm (e.g., in the range of 157 nm, in the EUVrange of wavelengths, such as 13 nm).

It is especially desirable to use radiation of a particularly shortwavelength, because the optical resolution of a projection objective isgenerally in proportion to the operating wavelength being used. This isthe reason why with the use of shorter wavelengths, projectionobjectives are capable of resolving smaller structures of an image thancan be resolved by projection objectives that are of the same kind butuse longer wavelengths.

The illumination system 2120 comprises optical components which producea collimated light beam with a largely homogeneous intensity profile.The illumination system 2120 further comprises optical arrangements thatserve to direct the light bundle 2122 onto the mask 2140. In someembodiments, the illumination system 2120 further includes componentswhich produce a specific polarization profile of the ray bundle.

The image plane 2102 has a distance L from the object plane 2103 whichis also referred to as the overall length B of the projection objective2101.

In the examples of embodiments that are described herein, the overalllength is in the range from one meter to about three meters (e.g., inthe range from about 1.3 m to 2.5 m).

In certain examples of embodiments, the overall length is shorter than 2m (e.g., shorter than 1.9 m, shorter than 1.8 m, shorter than 1.7 m,shorter than 1.6 m, shorter than 1.5 m).

The projection objective 2101 has an imaging factor that applies to theratio of the dimensions of the field in the object plane 2103 inrelation to the corresponding dimensions of the projected field in theimage plane 2102. Typically, the projection objectives that are used inlithography systems are reducing projection objectives, meaning that thedimensions of the image are smaller than those of the object. In someexamples, it is therefore possible for the projection objective 2101 toproduce a field in the image plane 2102 with image dimensions that arereduced by a factor of 2 to 10 (e.g., by a factor 4 or 5), in comparisonto the dimensions in the object plane 2103. However, it is also possibleto develop projection objectives which provide a magnified image or animage of the same size as the object.

FIG. 1 c shows the marginal rays 2152 of the light bundle which projectsthe object into the image plane 2102. The marginal rays 2152 define acone of rays.

The angle of the cone of rays is related to the image-side numericalaperture (NA) of the projection objective 2101. The image-side numericalaperture can be expressed as NA=n₀·sin Θ_(NA), wherein no stands for therefractive index of the medium that lies adjacent to the substrate 2150.This medium can be for example air, nitrogen, water, or a vacuum. Thesymbol Θ_(NA) stands for the angle that is defined by the marginal raysof the projection objective 2101.

In general, projection objectives 2101 have a relatively large numericalaperture NA on the image side. To give an example, the image-sidenumerical aperture NA of the projection objective 2101 in someembodiments is larger than 0.15, in particular more than 0.20, and evenlarger than 0.25. The optical resolution of the projection objective2101 generally varies as a function of the wavelength and the image-sidenumerical aperture NA.

The relationship between the resolution of a projection objective andthe wavelength and the image-side numerical aperture can be estimatedwith the formula

${R = {k \cdot \frac{\lambda}{NA}}},$wherein R stands for the minimal resolution of the projection objectiveand k is a dimension-less factor which is referred to as process factor.The process factor k varies as a function of different factors, forexample the polarization properties of the image projection or theselected modes of illumination. The process factor k lies typically inthe range from 0.4 to 0.8 but can also be below 0.4 or above 0.8 forspecial applications.

The projection objective 2101 in some embodiments has a relatively highresolution, meaning that the numerical value of R is relatively small.For example, the resolution R can have a value of 150 nm or less (e.g.,130 nm or less, 100 nm or less, 75 nm or less, 50 nm or less, 40 nm orless, 35 nm or less, 32 nm or less, 30 nm or less, 28 nm or less, 25 nmor less, 22 nm or less, 20 nm or less, 18 nm or less, 15 nm or less, 14nm or less, 13 nm or less, 12 nm or less, 11 n nm or less, 10 nm orless). The quality of an image that is formed by the projectionobjective 2001 can be quantified in different ways.

For example, images can be characterized or their quality level can bequantified based on the measured or calculated deviations of the imagefrom an ideal image point. These deviations are generally referred to asaberrations. A measure that is used to quantify the aberrations of awave front from the ideal or desired form is known as theroot-mean-square wave front aberration or RMS value W_(RMS). Adefinition of W_(RMS) is given for example in “Handbook of Optics”, Vol.1, 2^(nd) part, edited by Michael Bass (McGraw Hill), Inc. 1995, on page35.3. As a general rule, the lower the W s-value for an objective, theless the wave front deviates from the desired or ideal form, and thebetter is the quality of the image.

In some embodiments, the projection objective 2101 has very smallW_(RMS)-values of the image in the image plane 2102. For example, aprojection objective 2101 can have a W_(RMS)-value of about 0.1·λ orless (e.g., 0.07·λ, less than 0.07·λ, less than 0.06·λ, less than0.05·λ, less than 0.045·λ, less than 0.04·λ, less than 0.035·λ, lessthan 0.03·λ, less than 0.025·λ, less than 0.02·λ, less than 0.015·λ,less than 0.01·λ, less than 0.008·λ, and less than 0.006·λ).

Another measure that can be used to evaluate the quality of an image iscurvature of the image field, referred to as field curvature. The fieldcurvature is defined as the peak-to-valley variation in the axialposition of the focal plane as a function of the field point, in otherwords the maximum field variation of the focal plane. In someembodiments, the projection objective 2101 has a relatively small imagefield curvature for images in the image plane 2102. To mention examples,the projection objective 2101 has an image field curvature of less than20 nm (e.g., less than 15 nm, less than 12 nm, less than 10 nm, lessthan 9 nm, less than 8 nm, less than 7 nm, less than 6 nm, less than 5nm, less than 4 nm, less than 3 nm, less than 2 nm, less than 1 nm).

Another measure that can be used to evaluate the optical performance ofa projection objective is the distortion. The distortion is defined asthe maximum absolute value of the field-point-dependent deviation of theimage point from the ideal position of the image point in the imageplane. The projection objective in some examples has a relatively smalldistortion of 10 nm or less (e.g., 9 nm or less, 8 nm or less, 7 nm orless, 6 nm or less, 5 nm or less, 4 nm or less, 3 nm or less, 2 nm orless, 1 nm or less).

The projection objective can be a catoptric projection objective or acatadioptric projection objective. A catoptric projection objective hasexclusively reflective optical element such as for example mirrors.Catadioptric systems have reflective as well as refractive opticalelements.

If the objective is configured as a catoptric system, it contains amultitude of mirrors which are arranged so that the radiation thattravels from a mask 2140 to a substrate 2150 is reflected in such a waythat an image of the mask 2140 is formed on the surface of the substrate2150. Special design versions of a projection objective are configuredas described in the following paragraphs. Generally speaking, thenumber, size and structure of the mirrors is determined by the desiredoptical properties of the projection objective 2101 and the physicalboundary conditions of the projection exposure apparatus 2100.

The number of mirrors in the projection objective 2101 can vary.Typically, the number of mirrors is tied to different requirements thatare imposed on the optical properties of the objective.

In certain embodiments, the projection objective 2101 has at least twomirrors (e.g., at least four mirrors, at least five mirrors, at leastsix mirrors, at least seven mirrors, at least eight mirrors). In certainembodiments of the disclosure in which mirrors of the objective arearranged between the object plane and the image plane, the projectionobjective 2101 has an even number of mirrors, for example four, six oreight mirrors.

The projection objective 2101 generally comprises one or more mirrors ofpositive optical refractive power. This means in other words that thereflecting part of the mirror has a concave surface and is accordinglyreferred to as a concave mirror or concave-shaped mirror. The projectionobjective 2101 can contain two or more, for example three or more, andin particular four or more concave mirrors. The projection objective2101 can also include one of more mirrors that have negative opticalrefractive power. This means that one or more of the mirrors have areflecting portion with a convex surface. Mirrors of this kind are alsocalled convex mirrors or convex-shaped mirrors. In some embodiments, theprojection objective 2101 can have two or more, in particular three ormore, and most particularly four or more convex mirrors.

In certain embodiments, the mirrors are arranged in the projectionobjectives 2101 in such a way that the radiation that originates fromthe object plane 2103 forms one or more intermediate images.

Embodiments of the disclosure that have one or more intermediate imagescontain two or more pupil planes. In some embodiments, an aperture stopis arranged in a physically accessible way in at least one of thesepupil planes.

The mirrors are generally configured in such a way that a largeproportion of the light of the operating wavelength of the projectionobjective that falls on the mirror surface at an angle or within anangular range is substantially being reflected. The mirrors can also beconfigured so that of the radiation with a wavelength λ that falls onthe mirror surface, for example more than 50% is reflected (e.g., morethan 60% is reflected, more than 70% is reflected, more than 80% isreflected, more than 90% is reflected). In some embodiments, the mirrorsare coated with a stack of multiple layers, a so-called multi-layerstack in which the layers consist of different materials and the stackis designed so that is substantially reflects the radiation of thewavelength λ that falls on the surface. Every coating film of the stackhas an optical thickness of approximately λ/4. The multi-layer stackscan comprise 20 or more layers (e.g., 30 or more layers, 40 or morelayers, 50 or more layers). The multi-layer system consists for exampleof sets of multiple alternating layers consisting of molybdenum andsilicon or molybdenum and beryllium, in order to form mirrors thatreflect radiation in the range of wavelengths from 10 nm to 30 nm, forexample radiation with a wavelength λ of 13 nm or 11 nm.

The mirrors in certain embodiments are made of quartz glass and coatedwith a single layer of aluminum. The latter, in turn, is overlaid withdielectric layers that comprise materials such as MgF₂, LaF₂, Al₂O₃, forexample for wavelengths of about 193 nm.

In general, the part of the radiation that is reflected by a mirrorvaries as a function of the angle of incidence of the radiation on themirror surface. As the image-producing radiation is propagated along amultitude of different paths in a catoptric projection objective, theangle of incidence of the radiation can vary between mirrors. This isillustrated in FIG. 1 d which shows a part of a mirror 2300 in asectional view along the meridional plane. The mirror 2300 contains aconcave reflective mirror surface 2301. The image-producing radiationwhich reaches the surface 2301 along different paths includes forexample the paths that are represented by the rays 2310, 2320, 2330. Therays 2310, 2320, 2330 fall on a part of the mirror surface 2301. Thesurface-normal directions on the surface of the mirror vary in this partof the mirror surface 2301 and are represented by the lines 2311, 2321and 2331 for the points of incidence of the rays 2310, 2320 and 2330.The rays 2310, 2320 and 2330 meet the surface under the angles Θ₂₃₁₀,Θ₂₃₂₀ and Θ₂₃₃₀, respectively.

For every mirror in the projection objective 2100, it is possible torepresent the angles of incidence of the image-producing rays in amultitude of ways. One possible form of representation is through therespective maximum angle of the rays that fall on each mirror in themeridional section of the projection objective 2101. This maximum angleis referred to as Θ_(max). In general, the angle Θ_(max) can varybetween different mirrors of the projection objective 2101. In certainembodiments of the disclosure, the overall maximum value (Θ_(max))_(max)for all mirrors of the projection objective 2101 is 60° or less (e.g.,55° or less, 50° or less, 45° or less). In some examples, the overallmaximum angle (Θ_(max))_(max) is relatively small, for example 40° orless (e.g., 35° or less, 30° or less, 25° or less, 20° or less).

As another possibility, the incident light on a mirror can becharacterized through the angle of incidence on each mirror in themeridional section for the principal ray of the central field point ofthe field to be illuminated in the object plane. This angle is referredto as Θ_(CR). Concerning the principal ray angle Θ_(CR), reference isalso made to what has been said hereinabove in the introductory part. Itis again possible to define a maximum angle Θ_(CR(max)) in theprojection objective as the maximum principal ray angle of the centralfield point. This angle Θ_(CR(max)) can be relatively small, for examplesmaller than 40° (e.g., smaller than 35°, smaller than 30°, smaller than25°, smaller than 20°, smaller than 15°).

Each mirror in the projection objective 2101 can further becharacterized by a range of the angles of incidence in the meridionalsection of the projection objective 2101. The range within which theangle Θ varies on each mirror is referred to as ΔΘ. The range ΔΘ foreach mirror is defined as the difference between an angle Θ_((max)) andan angler Θ_((min)), wherein Θ_((min)) stands for the minimum angle ofincidence of the image-forming rays that fall on a mirror surface in themeridional section of the Projection objective 2101, and Θ_((max))stands for the maximum angle of the incident image-forming rays on amirror surface, as has already been defined above. The range ΔΘgenerally varies between mirrors in the projection objective 2101 andcan be relatively small for some mirrors, for example smaller than 25°(e.g., smaller than 20°, smaller than 15°, smaller than 10°). On theother hand, ΔΘ can be relatively large for other mirrors in theprojection objective 2101. For example, ΔΘ can be 20° or larger, inparticular 25° or larger (e.g., 30° or larger, 35° or larger, 40° orlarger). In some embodiments, the maximum value ΔΘ_(max) for all rangesΔΘ, i.e. the maximum over all mirrors of the projection objective 2101for the respective range of variation ΔΘ on each mirror can berelatively small, for example smaller than 25° (e.g., smaller than 20°,smaller than 15°, smaller than 12°, smaller than 10°, smaller than 8°).

FIG. 1 e shows an example of a mirror 2660 of the type used in theprojection objective. The mirror 2660 has the shape of a ring segment,i.e., a segment of a circular mirror 2670 with a diameter D. The mirror2660 has a maximum dimension M_(x) in the x-direction. In examples ofembodiments, the dimension M_(x) can be 800 mm or less (e.g., 700 mm orless and, 600 mm or less, 500 mm or less, 400 mm or less, 300 mm orless, 200 mm or less, 100 mm or less).

The mirror 2660 is symmetric relative to the meridional section 2675.The meridional plane is defined by the y- and z-axes of the localcoordinate system. The mirror 2660 has a dimension M_(y) along themeridional line 2675, which can be smaller or larger than M_(x). Incircular mirrors, for example in mirrors that are arranged in anaperture stop plane, the dimensions M_(x) and M_(y) are equal, i.e.,M_(x)=M_(y). In some embodiments, M_(y) lies in the vicinity of 0.1M_(x) (e.g., 0.2 M_(x) or more and, 0.4 M_(x) or more, 0.5 M_(x) ormore, 0.6 M_(x) or more, 0.7 M_(x) or more, 0.8 M_(x) or more, 0.9 M_(x)or more). On the other hand, M_(y) in certain embodiments can be equalto 1.1 M_(x) or larger (e.g., 1.5 M_(x) or larger, in the range from 2M_(x) to 10 M_(x)) M_(y) can be about 800 mm or less (e.g., 700 mm orless, 600 mm or less, 500 mm or less, 400 mm or less, 300 mm or less,200 mm or less, 100 mm or less).

The mirrors of the projection objective can be arranged so that theoptical axis 2105 intersects the mirror, but also in such a way that theoptical axis 2105 does not intersect the mirror.

Depending on the design, the projection objective 2100 can generallycomprise mirrors of different shapes and sizes. In some example, themaximum dimension D of each mirror of the projection objective can be1000 mm or less, in particular 900 mm or less (e.g., 800 or less, 700 mmor less).

In general, the shape of the field of the projection objective 2101 canvary. FIG. 1 f shows a ring segment 2700, also referred to as a ringfield. The ring segment 2700 can be characterized by an x-dimensionD_(x), a y-dimension D_(y), and a radial dimension D_(r). D_(x) andD_(y) are the dimensions of the field as measured, respectively, in thex-direction and in the y-direction. The amounts for these dimensionswill be named in the following description. For example in a field of26×2 mm² in the image plane, the dimension D_(x) is 26 mm and D_(y) is 2mm. The dimension D_(r) represents the ring radius measured from theoptical axis 2105 to the inner border of the field 2700. The ring fieldsegment 2700 is symmetric relative to a plane indicated by the line2710, which is parallel to the y/z-plane. In general, D_(x), D_(y) andD_(r) vary in magnitude, depending on the design of the projectionobjective 2101. Typically, D_(x) is larger than D_(y). The relativesizes of the field dimensions or field measurements D_(x), D_(y) andD_(r) in the object plane 2103 and in the image plane 2102 vary as afunction of the magnification or reduction ratio of the projectionobjective 2101. In some examples, D_(x) in the image plane 2102 isrelatively large, for example larger than 1 mm (e.g., larger than 3 mmand, larger than 4 mm, larger than 5 mm, larger than 6 mm, larger than 7mm, larger than 8 mm, larger than 9 mm, larger than 10 mm, larger than11 mm, larger than 12 mm, larger than 13 mm, larger than 14 mm, largerthan 15 mm, larger than 18 mm, larger than 20 mm, larger than 25 mm,larger than 30 mm). The dimension D_(y) in the image plane 2102 can liein the range from 0.5 mm to 5 mm (e.g., up to 1 mm, up to 2 mm, up to 3mm, up to 4 mm). Typically, D_(r) in the image plane 2102 is in therange from 10 mm to 50 mm (e.g., 15 mm or more or, 20 mm or more, 25 mmor more, 30 mm or more).

Generally speaking, for other field shapes such as for example arectangular field, the projection objective 2101 can have a maximumfield dimension or field measurement in the image plane 2102 of morethan 1 mm (e.g., more than 3 mm and, more than 4 mm, more than 5 mm,more than 6 mm, more than 7 mm, more than 8 mm, more than 9 mm, morethan 10 mm, more than 11 mm, more than 12 mm, more than 13 mm, more than14 mm, more than 15 mm, more than 18 mm, more than 20 mm, more than 25mm, more than 30 mm). FIG. 1 e further shows the central field point Z.The central field point Z defines the origin of a local x-y-z coordinatesystem. In scanning microlithography systems, the y-direction generallyindicates the scanning direction.

In general, the projection objective 2101 can be optimized and analyzedby using commercially available optical design programs such as ZEMAX,OSLO, Code V. After the wavelength, the field size and the numericalaperture have been defined at the outset, the optical properties can beoptimized which are needed in a projection objective, such as forexample the wave front aberration, the telecentricity, the uniformity aswell as the distortion and the image field curvature. In the following,examples for realizing the disclosure will be described in detail withoptical data. All optical data presented herein are given in Code Fformat.

FIG. 1 g gives a detailed illustration of a state-of-the-artmicrolithography projection exposure apparatus as disclosed in PatentApplication Publication US 2005/088760. The projection objective 1 has anegative back focus. The illumination system includes a primary lightsource 3 and a light-collecting optical element, a so-called collector5. The collector 5 is a grazing-incidence collector. The radiationemitted by the light source is filtered by means of the spectral filterelement 7 together with the aperture stop 9, so that behind the aperturestop there is only usable radiation of, e.g., 13.5 nm wavelength. Thespectral filter in the form of a grid element diffracts the light thatfalls on the grid element in different directions, for example in thefirst-order diffraction. The aperture stop is arranged in or near theintermediate image 11 of the primary light source 3 in the first-orderdiffraction. The projection exposure apparatus further includes a firstfacetted optical element 13 with first facets, so-called field rasterelements which are configured as small facet mirrors, and a secondoptical element 15 with second facets, so-called pupil raster elements.The first optical element 13 which comprises the field facets breaks upthe incident light bundle 17 which arrives from the primary light source3 into a multitude of light bundles. Each of the light bundles isfocused and forms a secondary light source at or near the place wherethe second optical element 15 with pupil raster elements is arranged.

If the field raster elements have the shape of the field to beilluminated, as shown in the illustrated examples, it is not necessaryto provide a mirror for the shaping of the field.

In the object plane 20 of the microlithography projection exposureapparatus, a reticle is arranged on a transport stage 19. The reticlewhich is arranged in the object plane 20 is projected by mans of theprojection objective 1 onto a light-sensitive substrate 22, for examplea wafer, which is likewise arranged on a carrier stage 23. Theprojection objective shown here comprises six mirrors, i.e. a firstmirror S1, a second mirror S2, a third mirror S3, a fourth mirror S4, afifth mirror S5, and a sixth mirror S6, in an arrangement that iscentered in relation to a common optical axis HA. The projectionobjective 1 has a negative back focus of the entry pupil. The secondfacetted optical element 15 of the illumination system is arranged in ornear the plane of the associated mirrored entry pupil RE.

As can be seen clearly in FIG. 1 g, with the configuration of thestate-of-the-art projection objective, the light-ray path crosses overitself between the illumination system and the projection objective, andconsequently this system is not amenable to a modular configurationwhich would allow a separation of the illumination system from theprojection objective.

FIG. 1 h represents the ray path for many of the embodiments ofprojection objectives that are presented herein, which have a negativeback focus in the area of the object plane 51 of the projectionobjective and of the mirror S1 that comes first in the light path fromthe object plane to the image plane, with the latter not being shown inFIG. 1 h. The symbol CRE in FIG. 1 h identifies the incident principalray falling on the first mirror, and CRR represents the reflectedprincipal ray belonging to one and the same field point, for example thecentral field point, of the object field. As can be seen in FIG. 1 h, insome embodiments of the disclosure the principal ray CRE of the incidentlight bundle in the meridional plane of the projection objective liesbetween the principle ray of the light bundle CRR that is reflected fromthe surface of the mirror S1 and the optical axis HA of the projectionobjective.

Also shown in FIG. 1 h is the local x-y-z coordinate system, the normaldirection NO that is perpendicular to the object plane 51 in which anobject field is formed, and the positive principal ray angle γ underwhich the principal ray CRE is reflected by an object (not shown) in theobject plane 51.

FIG. 2 a illustrates a first embodiment of a six-mirror projectionobjective which has a negative back focus of the entry pupil, but whichis amenable to a modular design configuration of a microlithographyprojection exposure apparatus. The objective according to FIG. 2 a hasan object plane 100, an image plane 102, a first mirror S1, a secondmirror S2, a third mirror S3, a fourth mirror S4, a fifth mirror S5, anda sixth mirror S6.

As can be clearly seen, no intermediate image is formed in the lightpath from the object plane 100 to the image plane 102 in the projectionobjective shown in FIG. 2 a. The objective has only a single aperturestop plane 104 which in the illustrated example is located on the fifthmirror, i.e. in the posterior part of the objective which consists ofthe fifth mirror S5 and the sight mirror S6. By arranging the aperturestop plane 104 in this way, it becomes possible to arrange the mirrorsS1 and S2 in the anterior part of the objective at a large distance fromthe optical axis HA. If a projection objective of this kind is used in aprojection exposure apparatus with a reflective object in an objectplane, the arrangement where the mirrors in the anterior part of theobjective have a large distance from the optical axis makes it possibleto place components of the illumination system, specifically a facettedoptical element of the illumination system, in this space on the opticalaxis of the projection system and thus in or near the mirrored entrypupil RE. The projection objective illustrated in FIG. 2 a has animage-side numerical aperture of NA=0.25 and a reduction ratio of 4. Thefield size of the field that is projected into the image plane is 2×26mm², which means that the dimension D_(y) of the field measured alongthe y-axis (see FIG. 1 e) is 2 mm, and the dimension D_(x) is 26 mm. Inthe example shown in FIG. 2, the principal ray CRE of the incident raybundle falling on the mirror surface of the first mirror S1 lies on themeridional plane between the reflected principal ray CRR associated withthe same field point of the reflected ray bundle and the optical axis HAof the projection objective. Further shown is the mirrored entry pupilRE of the projection objective, which lies in the plane 103. The pointof intersection with the optical axis of the principal ray CR of thelight beam 105 propagating from the object plane to the image plane isidentified by the label “CROSS”. According to the disclosure, this pointof intersection labeled CROSS lies in the posterior part of theobjective in the meridional section between the plane 103 in which themirrored entry pupil RE lies and the plane that contains the point ofintersection CROSS. The projection objective in the figuration of FIG. 2a has a resolution of 22 nm, an image-side wave front aberration RMS of0.008λ, an image-side field curvature of 7 nm and a distortion of 2.5nm. The objective has no intermediate image, and it possesses anaccessible aperture stop plane 104. As described above, the aperturestop B is formed in the accessible aperture stop plane 1043 which isalso at the same time a pupil plane and which includes the point ofintersection CROSS—on the fifth mirror. FIG. 2 a further shows thedistance A1 of the plane 104 containing the point of intersection CROSSfrom the object plane 100 along the optical axis HA as well as thedistance A2 of the plane 103 containing the mirrored entry pupil RE fromthe object plane 100. The two distances conform to the rule A2<A1. Alsoshown in FIG. 2 a are a first sub-objective (SUBO1) and a secondsub-objective (SUBO2), where the second sub-objective (SUOBO2) includesthe aperture stop B.

It can further be seen in FIG. 2 a that the projection objective issubdivided into two partial systems, i.e. a first partial system PART1and a second partial system PART2. The first partial system PART1 withthe mirrors S1 and S2 has a distance DIS along the optical axis HA fromthe second partial system PART2 with the mirrors S3, S4, S5 and S6.

The distance DIS is defined as the distance between the back surface ofthe mirror in the first partial system PART1 with the largest distancefrom the object plane 100 and the back surface of the mirror in thesecond partial system PART2 with the largest distance from the imageplane 102. In the present case, it is the distance between the backsurface of the first mirror S1 and the back surface of the fourth mirrorS4.

The distance between the object plane 100 and the image plane 102 is1500 mm, the maximum mirror diameter, i.e. the maximum dimension M_(y)of all mirrors as measured in the meridional section is 131 mm, and themaximum mirror diameter of all mirrors as measured in the x-direction,i.e., the maximum dimension M_(x), is 370 mm.

In the sequence from the first mirror S1 to the second mirror S2, thethird mirror S3, the fourth mirror S4, the fifth mirror S5, and thesixth mirror S6, the respective mirror curvatures are N-P-P-N-N-P, whichmeans convex-concave-concave-convex-convex-concave.

The principal ray angle γ in the first embodiment, i.e. the angle of theprincipal ray CR associated with the central field point of the field inthe object plane 100 is γ=7° in relation to the surface-normaldirection. The distance of the central field point from the optical axisis 132 mm. Based on these data, it can be calculated that the mirroredentry pupil RE has a maximum distance of 1075 mm from the object plane.The optical data of the embodiment shown in FIG. 2 a are listed in CodeV format in Table 1 which is attached as FIG. 2 b. The nomenclature oftables in Code V format are well understood by a man skilled in the art.In the Code V tables for reflective systems thickness mean in thecontext of mirror systems the thickness of the air space between twoneighboring optical surfaces, i.e., the distance along the optical axisbetween two optical surfaces that directly follow each other in thelight path

FIG. 3 a illustrates a second embodiment of a six-mirror projectionobjective according to the disclosure without intermediate image andwith negative back focus of the entry pupil. This embodiment has animage-side numerical aperture NA of 0.30 and a field size D_(y)×D_(x) of2×26 mm² as well as a reduction ratio of 4×. The image-side wave frontaberration is 0.03λ, the image-side field curvature 18 nm, and thedistortion is 4 nm. In the sequence of mirrors from the object plane tothe image plane the mirror curvatures follow each other as N-P-P-N-N-P,i.e. convex-concave-concave-convex-convex-concave. The projectionobjective includes an accessible aperture stop 104. The aperture stop Bis arranged in the accessible aperture stop plane 104 on the fifthmirror. The aperture stop plane is at the same time also a pupil planewhich contains the point of intersection CROSS of the principal ray CRwith the optical axis HA. The distance from the object plane 100 to theimage plane 102 is 1600 mm, the maximum dimension M_(y) in themeridional section for all mirrors is 176 mm, and the maximum mirrordiameter, i.e. the maximum dimension M_(x) measured in the x-directionfor all mirrors is 459 mm.

The principal ray angle at the central field point is γ=7° at theobject, and the distance of the central field point from the opticalaxis is 159 mm. The mirrored entry pupil RE in the plane 103 has anaxial distance A2 of 1295 mm from the object plane 100. Components thatare analogous to those in FIG. 2 a have the same reference symbols. Asin FIG. 2 a, the principal ray CRE of the incident ray bundle falling onthe surface of the first mirror S1 lies in the meridional sectionbetween the optical axis and the principal ray CRR of the ray bundleassociated with the same field point that is reflected from the surfaceof the first mirror. The point of intersection CROSS of the principalray CR of the central field point with the optical axis HA of theprojection objective is geometrically located between the plane 103 withthe mirrored entry pupil RE of the projection objective and the imageplane 102. The distance along the optical axis between the plane 104which contains the point of intersection CROSS and the object plane islabeled with A1, and the distance along the optical axis between theplane 103 which contains the mirrored entry pupil and the object planeis labeled with A2. In the present embodiment, A2 is smaller than A1,due to the location of the point of intersection CROSS between themirrored entry pupil RE and the image plane.

As can be seen in FIG. 3 a, the principal ray has on its light path fromthe object plane 100 to the image plane 102 exactly one point ofintersection CROSS with the optical axis. Therefore, according to thedisclosure, all points of intersection of the projection objective liebetween the plane 103 of the mirrored entry pupil RE and the image plane102.

The second embodiment likewise comprises two partial systems, i.e. afirst partial system PART1 and a second partial system PART2, at adistance DIS from each other.

The system data in Code V format for the second embodiment according toFIG. 3 a are listed in Table 2 in FIG. 3 b.

FIG. 4 shows a microlithography projection exposure apparatus with aprojection objective according to the embodiment of FIG. 2 a and FIG. 2b. As can be seen in FIG. 4, there is no cross-over between the raypattern of the projection system and the ray pattern of the illuminationsystem, which stands in contrast to the system of FIG. 1 g. In otherwords, the illumination ray bundle 211 on its light path from thenext-to-last optical component 206 to the object plane 212 where theobject such as a reticle is arranged does not cross over theimage-producing ray pattern 213 which proceeds in the projectionobjective from the object plane 212 to the image plane 214, except forthe necessary cross-over which occurs at the reticle. The meridionalplane in the present case is the plane of the drawing, which includesthe optical axis HA. The optical elements of both parts of a projectionexposure apparatus, i.e. the illumination system and the projectionsystem, are arranged in separate design envelopes. The optical elementsof the illumination system are arranged in a first design space B1,while the optical elements of the projection system are arranged in asecond design space B2. The first design space B1 is separated forexample by a wall W from the second design space B2. Due to theseparation between two different design spaces B1 and B2, one obtains amodular design structure for the projection exposure apparatus. As canfurther be seen, there is enough space available for example for theinstallation of the pupil facet mirror 208 of a double-facettedillumination system.

The projection exposure apparatus shown in FIG. 4 which operates atwavelengths larger than 100 nm is an example of a projection exposureapparatus in particular for EUV lithography with an image-side numericalaperture of NA≧0.25 (e.g., NA>0.26) and with fewer than 10 opticalelements that have a normal-incidence reflection, wherein a cross-overof rays between the respective ray pattern of the illumination systemand the projection system is avoided.

The illumination of the microlithography projection exposure apparatusshown in FIG. 4 includes a light source 200, a nested grazing-incidencecollector 202, as well as a spectral grid filter 204. The spectral gridfilter is identified by the reference symbol 204; the aperture stop thatfollows the spectral grid filter is not shown in the present example.The spectral grid filter is followed in the ray path by the two facettedoptical elements of the double-facetted illumination system. The firstfacetted optical element, which comprises a multitude of first rasterelements, so-called field facets, is identified by the reference symbol206, and the second facetted optical element, the so-called pupil facetmirror, is identified by the reference symbol 208. The second facettedoptical element 208 is arranged in the plane of the mirrored entry pupilRE of the projection objective. Due to the negative back focus of theentry pupil of the projection objective 210, the mirrored entry pupil RElies to the image side of the object plane 212 in which for example thereflective reticle is arranged. The object plane is identified as 212,the image plane as 214, the optical axis as HA, the first mirror as S1,the second mirror as S2, the third mirror as S3, the fourth mirror asS4, the fifth mirror as S5, and the sixth mirror as S6.

In the following embodiments of microlithography projection objectiveswith negative back focus of the entry pupil according to FIGS. 5 a and 5b and FIGS. 6 a and 6 b, a free space is created in the microlithographyprojection exposure apparatus in the area of the mirrored entry pupil REthrough a design concept that provides for a cross-over of ray bundlesin the anterior part of the objective. The anterior part of theobjective is that part of the projection objective which comprises themirrors which are arranged nearest to the object plane. With this kindof a design it is possible to arrange in particular the second mirror S2of the six-mirror objective shown in the examples of FIG. 5 a and 6 a ata large distance from the optical axis HA of the projection objective.The cross-over of rays in the meridional plane which contains theoptical axis HA of the projection objective occurs in the illustratedexamples of FIG. 5 a, 5 b and 6 a, 6 b between the ray bundle whichproceeds from the object plane 300 to the first mirror S1 and the raybundle which proceeds from the second mirror S2 to the third mirror S3.The aperture stop B is arranged in the ray path from the second mirrorS2 to the third mirror S3. Due to the large distance of the mirrors inthe anterior part of the objective from the optical axis, it is possibleto design a microlithography projection exposure apparatus in which theray pattern of the illumination system does not cross over the raypattern of the projection objective in the meridional plane, so that aseparation is possible between the illumination system and theprojection objective, as is the case for example in the system in FIG.4.

In the example shown in FIG. 5 a of a six-mirror projection objectivewith cross-over of the ray bundles in the anterior part of theobjective, an intermediate image ZW is formed between the fourth mirrorS4 and the fifth mirror S5. The first mirror is labeled as S1, thesecond mirror as S2, the third mirror as S3, and the sixth mirror in thelight path from the object plane 300 to the image plane 302 is labeledas S6. As was already the case in FIGS. 2 a and 3 a, FIG. 5 a shows onlythe usable portion of each mirror, with the usable portion of the firstmirror S1 labeled N1, the usable portion of the second mirror S2 labeledN2, the usable portion of the third mirror S3 labeled N3, the usableportion of the fourth mirror S4 labeled N4, the usable portion of thefifth mirror S5 labeled N5, and the usable portion of the sixth mirrorS6 labeled N6. The usable portion of a mirror is the area that meets therays of the light bundle which travel from the object plane to the imageplane. The embodiment according to FIG. 5 has an image-side numericalaperture of NA=0.25, a reduction ratio of 4×, and a field size of 2×26mm² of the field in the object plane, i.e. D_(y)=2 mm and D_(x)=26 mm.The principal ray angle of the central field point is γ=7° at the objectand the distance of the central field point from the optical axis is 93mm. As can be calculated from these data, the mirrored entry pupil REhas an axial distance of 757 from the object plane. The resolution is 22nm, the image-side wave-front aberration RMS is 0.006λ, the image-sidefield curvature is 1.5 nm, and the distortion is 6 nm. The mirrorsfollow each other in the sequence P-P-N-P-N-P, i.e.concave-concave-convex-concave-convex-concave. Two planes that areconjugate to the entry pupil, so-called pupil planes 312, 314, areformed in the system. The pupil plane 312 contains the point ofintersection CROSS1 of the principal ray CR with the optical axis, whilethe pupil plane 314 contains the point of intersection CROSS2. Theprojection objective is telecentric on the image side, so that the exitpupil lies at infinity. The system is distinguished by the fact that theexit pupil is not obscured. The term “exit pupil” means the image of theaperture stop produced by the partial objective that follows theaperture stop. An accessible aperture stop B is arranged in one of thetwo pupil planes 312, 314 which is accessible from at least one side ofthe projection objective. The aperture stop B in the present example isformed between the second and third mirrors. The maximum mirrordiameter, i.e. the maximum dimension M_(y) of all mirrors as measured inthe meridional section is 157 mm, and the maximum mirror diameter of allmirrors as measured in the x-direction, i.e., the maximum dimensionM_(x), is 389 mm. The maximum angle ΔΘ_(CR(max)) of the principal ray ofthe central field point for all mirrors is 16.4°, the maximum angle ofincidence Θ_(max(max)) on all mirrors in the meridional section is 21°.The maximum bandwidth ΔΘ_(max) of the angles of incidence in themeridional section on each mirror is 17.3° for all mirrors.

The distance between the object plane and the image plane is 1550 mm.

The optical data of the example of FIG. 5 a are listed in Code V formatin Table 3 which is found in FIG. 5 b.

FIG. 6 a illustrates a further embodiment of a six-mirror projectionobjective according to the disclosure with an intermediate image andwith negative back focus of the entry pupil. This embodiment has animage-side numerical aperture NA of 0.30, a reduction ratio of 4×, and afield size of 2×26 mm², i.e. D_(y)=2 mm and D_(x)=26 mm. The principalray angle at the central field point is γ=7° at the object, and thedistance of the central field point from the optical axis is 106 mm. Thedistance between the object plane 300 and the image plane 302 is 1520mm. The mirrored entry pupil RE has an axial distance of 754 mm from theobject plane. The system has a resolution of 18 nm, an image-side wavefront aberration RMS of 0.018λ, an image-side field curvature of 11 nm,and a distortion of 3.2 nm. The six mirrors follow each other from theobject plane to the image plane in the sequence P-P-N-P-N-P, i.e.concave-concave-convex-concave-convex-concave. Two pupil planes areformed in the light path from the object plane to the image plane, withone of the pupil planes being accessible. The aperture stop B isarranged in the accessible pupil plane. The aperture stop plane B isformed between the second and the third mirror. The maximum mirrordiameter, i.e. the maximum dimension M_(y) of all mirrors as measured inthe meridional section is 189 mm, and the maximum mirror diameter of allmirrors as measured in the x-direction, i.e., the maximum dimensionM_(x), is 423 mm. The maximum angle of incidence ΔΘ_(CR(max)) of theprincipal ray of the central field point for all mirrors is 19°, themaximum angle Θ_(max(max)) for all mirrors in the meridional section is24.1°, and the maximum range ΔΘ_(max) of the angles of incidence for allmirrors is 19.8°. Components that are analogous to those in FIG. 5 a areidentified by the same reference numerals. Common to both of the systemsof FIG. 5 a and FIG. 6 a is the spatial arrangement of the mirrors inthe anterior part of the objective along the optical axis, namely:

second mirror S2-fourth mirror S4-first mirror S1-third mirror S3.

Furthermore common to both of the systems of FIG. 5 a and FIG. 6 a, theaperture stop B is arranged between the second mirror S2 and the thirdmirror S3 in or near the pupil plane 312.

The optical data of the embodiment according to FIG. 6 a are listed inCode V format in Table 4 which is presented in FIG. 6 b.

The embodiment of FIG. 6 c represents an alternative to the systemsaccording to FIGS. 5 a and 6 a. The aperture stop B in the embodiment ofFIG. 6 c is located directly on the third mirror S3 in the pupil plane312. Placing the aperture stop on a mirror has the advantage that thepassing ray bundles are not separated as strongly from each other, sothat either the angles of incidence can be made smaller or, if theangles of incidence are kept the same, the overall length can be madeshorter, as is the case in this example. The system according to FIG. 6c has the further advantage that the angles of incidence are small onall mirrors. The spatial arrangement of the mirrors in the anterior partof the objective is as follows:

fourth mirror S4-second mirror S2-first mirror S1-third mirror S3.

Compared to the embodiment of FIG. 6 a, the positions of the mirrors S2and S4 along the optical axis have been switched.

FIG. 6 a shows a lens section of the foregoing system. The optical datafor this system are listed in Code V format in Table 5 which is shown inFIG. 6 d. The image-side numerical aperture of the system is NA=0.25,and the field size is 2×26 mm², wherein the field is configured as aring field segment. The reduction ratio of the system of FIG. 6 c is 4×,the image resolution is 22 nm, the RMS value for the image-sidewave-front aberration is 0.019λ, the image-side field curvature is 20nm, and the distortion is 0.8 nm. There are six mirrors in totalfollowing each other in the sequence PPNPNP, i.e.concave-concave-convex-concave-convex-concave. Overall, the system hastwo pupil planes 312, 314, one of which is accessible. The accessiblepupil plane is at the same time the plane in which the aperture stop Bis arranged. The aperture stop B is arranged on the third mirror. Thedistance between the object plane 100 and the image plane 102 is 1490mm, the maximum mirror diameter, i.e. the maximum dimension M_(y) of allmirrors as measured in the meridional section is 197 mm, and the maximummirror diameter of all mirrors as measured in the x-direction, i.e., themaximum dimension M_(x), is 464 mm, and the maximum angle of incidenceΔΘ_(CR(max)) of the principal ray of the central field point for allmirrors is 16.6°, the maximum angle Θ_(max(max)) for all mirrors in themeridional section is 19.2°, and the maximum range ΔΘ_(max) of theangles of incidence in the meridional section for all mirrors is 16.7°.

The following FIGS. 7 to 13 show advantageous embodiments ofmicrolithography projection exposure apparatus which are compatible withthe use of projection objectives with negative back focus of the entrypupil as disclosed herein, or as also described in US 2005/0088760.

In the illumination systems of the known state of the art, theprojection objectives mostly have a positive back focus of the entrypupil. In projection systems that have projection objectives with apositive back focus, the optical elements such as for example aholographic diffuser or also a second facetted optical component of anillumination system which is configured as a double-facetted systemcannot be placed in the entry pupil of the projection objective, but areprojected as images by means of optical elements into an entry pupilthat is located after the object plane in the light path from a lightsource to an image plane.

In a projection objective with a negative back focus of the entry pupil,the mirrored entry pupil is located to the image side of the objectplane in which for example the reflective reticle is arranged. In thesesystems, an optical integrator such as for example a diffuser, or alsothe pupil facet mirror in a double-facetted illumination system, can bearranged in or near the mirrored entry pupil RE.

A first embodiment of a system of this type is shown in FIG. 7. Thesystem according to FIG. 7 includes a projection objective 1000 withnegative back focus which has a first mirror S1, a second mirror S2, athird mirror S3, a fourth mirror S4, a fifth mirror S5, and a sixthmirror S6. Furthermore, the illumination system includes a nestedgrazing-incidence collector 1002 which is located in the light pathdownstream of the light source and receives the radiation of the lightsource 1004 in a half-space with a large aperture of NA≧0.7. Theillustrated collector in the schematic sketch has only two mirror shellswhich are rotationally symmetric relative to an axis of rotation,wherein two reflections occur at each of the shells. Of course, acollector with more than two shells and with more than two reflectionsper shell would likewise be conceivable. In the embodiment shown in FIG.7, a normal-incidence mirror 1008 is arranged in the light path from thelight source 1004 to the object plane 1006. Due to its multi-layeredcoatings, for example 40 to 70 Mo/Si coatings, the normal-incidencemirror 1008 functions as a narrow-band wavelength filter. The concept ofusing a normal-incidence mirror which due to its multi-layered coatingacts as a narrow-band wavelength filter belongs to the known state ofthe art. A mirror of this type can be moved into different positions, sothat different usable areas 1008.1 and 1008.2 can be positioned in theray path. In the present example, the move to a different locationoccurs by turning about the axis of rotation RA. The area of themulti-layered mirror 1008 that is moved out of the ray path can now becleaned, for example with a cleaning device. In addition, a spectralgrid filter can be put on the currently operative usable range 1008.1,1008.2 of the normal-incidence multi-layered mirror 1008. The way inwhich the state-of-the-art spectral filter removes light that is not ofthe usable wavelength is that the light of the light source falls on agrid which has at least one grid period in a grid plane that issignificantly larger than the usable wavelength (e.g., 150 to 200 timeslarger than the usable wavelength). If the usable wavelength is forexample around 13.5 nm, the periodicity of a binary grid acting in thismanner as a spectral filter is of the order of microns.

In the light path from the light source to the first facetted element, afirst intermediate image IMI is formed in the illumination ray path inthe illustrated embodiment. The optical components which are located inthe illumination ray path before the intermediate image IMI, i.e. thelight source 1004, the collector 1002, as well as the normal-incidencemirror 1008 are combined in a first space 1020. This first space 1020 isseparated from the rest of the components by a screen with only a singleopening 1022, which is substantially where the intermediate image IMI islocated. The first facetted optical component includes a mirror, theso-called field facet mirror 1024 with a large number of field facetswhich in the present example have the same shape as the field to beilluminated in the object plane. The field facets are thereforeconfigured with an arcuate shape. As an option for adjusting the settingof the illumination, the field facet mirror 1042 can be exchanged foranother field facet mirror. One possibility for the way in which onefacet mirror can be exchanged for another consists for example inputting field facet mirrors with different facet arrangements on acomponent that is capable of swiveling about an axis.

The field facet mirror 1024 disassembles the incident light bundle intoa large number of light bundles which are associated with the individualfield facets. Each light bundle associated with a field facet forms asecondary light source. Arranged at or near the location of thesecondary light source is the second facetted optical element, either inor close to the plane in which the mirrored entry pupil RE of theprojection objective is located. The second facetted optical element isalso referred to as pupil facet mirror 1026, and the raster elementsarranged on it are referred to as pupil facets. Like the field facetmirror, the pupil facet mirror 1026 also comprises 200 to 300 facettedelements, in this case pupil facets. The individual pupil facets can beconfigured to be switchable to allow an adjustment of the setting. Inthe area that is not being used, the pupil facet mirror can havecutouts, so that the path of light propagation is not impeded in theprojection objective. In order to suppress scattered light and keep itfrom entering the projection objective, there is a screen 1030 arrangedabove the facet mirror. As an alternative to a setting adjustment bymeans of a change in the channel assignment, it is also possible to setan aperture stop in place before the pupil facet mirror, which ishowever not illustrated in the present embodiment.

FIG. 8 shows an alternative configuration of an illumination system inwhich the optical integrator is configured as a diffuser. The lightsource in FIG. 8 is labeled with the reference numeral 2000. The lightsource 2000 is in this case a laser-plasma source with a horizontal pumpjet. In order to collect the part of the radiation that is emitted tothe rear of the radiation source, the arrangement includes anormal-incidence mirror 2004 which can be configured as a rotationalellipsoid. A possible alternative is to collect the light with more thanone collector, for example by using a dual collector. A dual collectorsystem is a normal-incidence collector system with two obliquelypositioned normal-incidence collector mirrors. A spectral grid filtercan be incorporated on the ellipsoid of the collector.

As a rule, the diffuser 2002 is configured as a diffuser mirror with 500to 1000 small mirror facets or, alternatively, as a holographic latticegrid.

As shown in FIG. 8, the illumination system is of a very compact designand consists merely of the light source 2000, the collector 2004, andthe diffuser 2002. In order to insulate the illumination system from theprojection objective with negative back focus of the entry pupil, thereare screens set up in the microlithography projection apparatus,optionally in the form of protective shields that can be cooled. Thescreen is labeled 2005. The projection objective is a six-mirrorprojection objective with a first mirror S1, a second mirror S2, a thirdmirror S3, a fourth mirror S4, a fifth mirror S5, and a sixth mirror S6.The diffuser 2002 in the form of an optical integrator in the systemshown in FIG. 8 is again arranged in or near the mirrored entry pupil REof the projection objective. Due to the limited designs space and due tothe angles of propagation, a setting of σ>1 is not possible in thesystems according to FIGS. 7 and 8.

The system of FIG. 8 has only 8 mirrors in the light path from the lightsource to the image plane of the projection objective where the objectto be illuminated is arranged, for example a wafer. However, thereflective reticle in the object plane was not counted in the number ofmirrors for the system according to FIG. 8. As an alternativepossibility in the system according to FIG. 8, the collector 2004 can bereplaced by an other collector, for example a grazing-incidencecollector. Even with a system of this kind with a grazing-incidencecollector, it is possible to specify an EUV projection exposureapparatus with nine or fewer mirrors in which an image-side numericalaperture of NA≧0.25 (e.g., NA≧0.3) is possible with a field size of 2×26mm².

In order to make a setting with σ>1 possible, it is advantageous to foldthe light path in the illumination system, for example by means of aplanar mirror. This is demonstrated in the system in FIG. 9. Componentsthat are analogous to those in the preceding example are labeled withthe same reference numerals. With the mirror 2008 the light path beforethe second optical element 2007 with second raster elements (not shown)is folded towards the reticle that is arranged in the object plane 2009and thus allows the first optical element 2006 with first rasterelements, i.e. the field facet mirror, to be located in an easilyaccessible space. The field facet mirror can be arranged on a support2010 that includes different field facet mirrors and can be swiveledabout an axis of rotation R1. The rotatable support can serve toexchange facet mirrors of equal configuration in order to clean theexchanged mirror in case of contamination. As another possibility, thesupport element for the facet mirror can also carry different facetmirrors, i.e. facet mirrors with different arrangements of rasterelements, so that different illumination settings can be realized by arotation about the axis R1. Combinations of these concepts are alsopossible.

As an option, the folding mirror 2008 can be a mirror with refractivepower. In the system shown in FIG. 9, the field facets are correlatedcrosswise with the respective pupil facets. This means that a fieldfacet which in the meridional section shown in FIG. 9 is located in theright-hand part of the field facet mirror is correlated with a pupilfacet in the left-hand part of the pupil facet mirror in the meridionalsection. As a consequence of this cross-over correlation, a constrictionof the light ray pattern, i.e. an intermediate image IMI1 of the lightsource, is formed in the light path from the first facetted opticalelement to the second facetted optical element. The constriction or theintermediate image IMI1 makes it possible to arrange a limited passageopening OP in a protective wall 2020 that separates the unit comprisingthe light source 2000, the collector 2004.1 and the first facettedoptical element 2006 from the unit that comprises the projectionobjective. Furthermore, an intermediate image IMI2 of the light sourceis also formed in the unit which comprises the light source 2000, thecollector 2004.1 and the first facetted optical element 2006.

FIG. 10 shows a further embodiment of a microlithography projectionexposure apparatus, in particular for wavelengths in the EUV range, inwhich the optical integrator is not located in the mirrored entry pupil.

The illumination system of this microlithography projection exposureapparatus includes a location-variant or a field dependent diffuser3006. This diffuser 3006 is designed in such a way that it does not needto be arranged in the plane of the mirrored entry pupil or in a planethat is conjugate to the latter, but that it can be arranged in almostany arbitrarily selected plane. The diffuser comprises a large number ofindividual mirror facets, optionally more than 1000, which havedeflecting angles that depend on the location where the diffuser isarranged in the illumination system, so that each facet receives lightfrom the light source and directs the light to an assigned discretepoint of a field (not shown) that is formed in the object plane 3007 ofthe illumination system. The discrete points in the object plane 3007are selected so that a field in the object plane 3007 is illuminated ina predetermined, for example arcuate, shape. The facet is furthermorearranged in such a way that, with the respectively assigned discretepoint of the field in the object plane, the facet will illuminate aspecific area in a pupil plane of the projection objective.

The diffuser 3006 is also referred to as a so-called specular reflector.It is distinguished in particular by the fact that the facets differ intheir sizes and positions as well as in their angles of inclination,where the angles of inclination are defined by the respectivelyassociated field points. The specular reflector optionally has asubstantially similar shape as the field to be illuminated. If the fieldto be illuminated has an arcuate shape, the specular reflector iskidney-shaped.

Due to the fact that the place for the location-variant or fielddependent diffuser 3006 can be arbitrarily chosen in the design of theillumination system, it is therefore possible to select an optimallocation for the diffuser 3006 independent of the layout of the rest ofthe system. The diffuser is arranged optionally in such way that thelayout of the system is optimized and the diffuser has an optimal size.A system with a location-variant or field dependent diffuser 3006 isillustrated in FIG. 10. The system of FIG. 10 includes a light source3000. The light of the light source is collected by means of a collectormirror 3002 and directed to a folding mirror 3004. The folding mirror3004 is necessary in order to separate the ray pattern of theillumination system from the ray pattern of the projection objective.The light falling on the mirror 3004 is reflected and sent to thelocation-variant or field dependent diffuser 3006. In contrast to thesystems shown previously, the location-variant or field dependentdiffuser 3006 is arranged in an arbitrarily selected plane, i.e. not inthe mirrored entry pupil plane or a conjugate plane of the latter, norin a field plane such as the object plane or a conjugate plane of thelatter.

In the light path from the light source 3000 to the diffuser, thenormal-incidence mirror 3004, which is placed before the diffuser 3006,has the task of acting as a filter for the light that falls on thelocation-variant or field dependent diffuser, as was described in thepreceding examples. Based on this filtering effect, the location-variantor field dependent diffuser receives only usable radiation, specificallyEUV light of a wavelength of 13.5 nm. This minimizes the radiationexposure load on the diffuser 3006, which has the result of a lowerthermal stress load and a lower contamination.

The normal-incidence mirror 3004 can optionally be configured as afree-form surface with an extra-axial conical component. The function ofa normal-incidence mirror of this kind is illustrated in FIG. 11 whichshows how due to the design of the normal-incidence mirror as afree-form surface, for example with an extra-axial conical component,the location-variant or field dependent diffuser 3006 with thekidney-shaped layout receives a kidney-shaped and thus largely loss-freeillumination.

In FIG. 11, the incident light bundle arriving from the light source islabeled 4000. The light bundle 4000 falls on the normal-incidence mirror3004 with a conical component as drawn in perspective in FIG. 11 and isreflected by the latter. The illumination pattern which this reflectionproduces in the plane where a location-variant or field dependentreflector or diffuser is arranged is labeled 4502. As can be clearlyseen in FIG. 11, the illumination in the plane where thelocation-variant or field dependent diffuser is arranged iskidney-shaped and consists of sub-pupils 4504 that are offset from eachother.

As an alternative to using a normal-incidence mirror, it is alsopossible to achieve a largely kidney-shaped illumination with agrazing-incidence mirror, for example with a shell of agrazing-incidence collector. Furthermore, the normal-incidence mirrorcan also have an optical function in addition to the illumination. Theoptical capability can be achieved by adding to the conical component aspherical, toroidal or generally aspherical component in order to giverefractive power to the mirror.

In the system of FIG. 10, the light of the diffuser 3006 is directedinto the object plane 3007 by means of a grazing-incidence mirror 3008.The object in the object plane is projected by means of the projectionobjective 3010 with six mirrors S1, S2, S3, S4, S5 and S6 into the imageplane 3022 where the object to be exposed, for example the wafer 3024,is arranged on a carrier 3026. The projection objective with negativeback focus has an optical axis HA.

With the system according to FIG. 10, a projection exposure apparatus ismade available which illuminates a field of 26×2 mm² with an image-sidenumerical aperture NA≧0.25 (e.g., NA≧0.3). The projection exposureapparatus has a maximum of 10 or fewer components at whichnormal-incidence reflections occur.

FIGS. 12 and 13 show alternative configurations of a microlithographyprojection exposure apparatus with a location-variant or field dependentdiffuser in the illumination system.

In the embodiment of FIG. 12, the location-variant or field dependentdiffuser 3006 is followed in the ray path by a light-collectingnormal-incidence mirror. Components that are analogous to those in FIG.10 are labeled with the same reference numerals. Putting thenormal-incidence mirror 3050 after the location-variant or fielddependent diffuser in the light path from a light source to an imageplane has in particular the advantage that the individual facets of thelocation-variant diffuser can be designed as planar mirrors. Thelight-collecting normal-incidence mirror 3050 produces a stronglymagnified image of the light source 3000 in the object plane 3007 wherethe reticle is arranged, while the location-variant diffuser on theother hand provides a superposition of a multitude of images of thelight source in the object plane 3007. The normal-incidence mirror 3050with refractive power is followed in the light path in the illustratedexample by a further mirror 3054, which is configured as a planarmirror. This mirror 3054 serves to fold the ray path. As a possiblealternative, the second normal-incidence mirror 3054 could also beprovided with refractive power, so that in combination with the firstnormal-incidence mirror 3050 the imaging scale ratio can be set to aspecified target value and the size of the location-variant diffuser canthus be controlled. This allows the location-variant diffuser 3006 to bedesigned very large, so that the individual facet elements on thelocation-variant diffuser are likewise of an appreciable size. Due tothe size of the location-variant diffuser 3006 and the individual facetsarranged on it, the thermal stress load can be strongly reduced, as hasbeen described above. The location-variant diffuser can also be formedon a curved carrier body.

In the embodiment of a system with a location-variant diffuser as shownin FIG. 12, the light of the light source 3000 is collected by adirection-reversing reflector 3060 which is configured in such a waythat a kidney-shaped area is illuminated on the location-variantdiffuser 3006. In this example, the direction-reversing reflectorprovides the largely arc-shaped illumination of the location-variantdiffuser 3006, instead of the normal-incidence mirror of the embodimentof FIGS. 10 and 11 with a free-form surface with an extra-axial conicalcomponent.

A particularly simple illumination system with a location-variant orfield dependent diffuser is shown in the example according to FIG. 13.Components that are analogous to whose in FIGS. 10 to 12 are labeledwith the same reference numerals. In the embodiment of FIG. 13, thelight of the light source is collected by a grazing-incidence collector3070 with a large number of mirror shells 3070.1, 3070.2. The lightreceived by the grazing-incidence collector is directed to alocation-variant or field dependent diffuser 3006 which is arrangedimmediately before the object plane 3007. The light that is reflected bythe location-variant or field dependent diffuser 3006 is directed to theobject plane 3007. By adding a mirror 3080 by means of which the lightreflected from the diffuser is folded back into the object plane 3007,the geometric efficiency of the system can be strongly enhanced. Theembodiment illustrated in FIG. 13 is distinguished in particular by thefact that it has a very simple design structure and that there is nointermediate image in the illumination light path of the illuminationsystem.

The projection objectives according to FIG. 12 as well as FIG. 13 areobjectives with a negative back focus of the entry pupil and six mirrorsS1, S2, S3, S4, S5 and S6.

The present disclosure provides for the first time a projectionobjective with negative back focus of the entry pupil which is designedin such a way that a cross-over between the illumination light path andthe image-projecting light path in a microlithography projectionexposure apparatus is largely avoided. This, in turn, makes it possibleto realize a modular design of the projection exposure apparatus.

In addition to the microlithography projection systems with negativeback focus of the entry pupil which allow a modular design of theforegoing description to be realized, the disclosure also providesmicrolithography projection systems with a small number of opticalcomponents and with an image-side numerical aperture NA≧0.25 (e.g.,A≧0.3) with a maximum dimension (D_(x), D_(y)) of a field on the imageside of more than 1 mm (e.g., more than 3 mm, more than 4 mm, more than5 mm, more than 6 mm, more than 8 mm, more than 10 mm, more than 12 mm,more than 15 mm, more than 20 mm, more than 25 mm). In a system of thistype, there are optionally ten or fewer normal-incidence reflections onoptical elements between the light source and the image plane, whereinthe reflection on the reflective object that is arranged in the objectplane, specifically the reflective reticle, is not counted. The size ofthe image field can be for example 2×26 mm². Particularly advantageouswithin this context are microlithography projection systems in whichonly one optical integrator is required. Among systems with only oneoptical integrator, a special advantage is held by those systems thatare distinguished by the fact that the optical integrator can be placedat an arbitrarily chosen location in the microlithography projectionexposure apparatus. Optical integrators which meet these requirementsare in particular so-called location-variant or field dependent opticalintegrators or specular reflectors.

1. A projection objective, comprising: an object plane in which anobject field is formed, an entry pupil and an exit pupil, a mirroredentry pupil in a mirrored entry pupil plane obtained by mirroring theentry pupil at the object plane, an image plane, an optical axis, atleast a first mirror and a second mirror, wherein the projectionobjective has a negative back focus of the entry pupil, and wherein aprincipal ray originating from a central point of the object fieldtraverses the objective from the object plane to the image plane andintersects the optical axis in at least one point of intersection,wherein each individual point of intersection has along the optical axisa respective first distance A1 from the object plane and the mirroredentry pupil plane has a second distance A2 from the object plane,wherein A2 is less than A1, and wherein the exit pupil is a non-obscuredexit pupil.
 2. The projection objective according to claim 1, whereinthe projection objective comprises at least four mirrors.
 3. Theprojection objective according to claim 2, further comprising anaperture stop located in an aperture stop plane.
 4. The projectionobjective according to claim 3, wherein: 1) a first number of mirrors,which in the light path from the object plane to the image plane liebefore the aperture stop plane, form a first sub-objective; and 2) asecond number of mirrors form a second sub-objective, wherein the secondsub-objective includes the aperture stop plane.
 5. The projectionobjective according to claim 4, wherein the second sub-objective has twomirrors.
 6. The projection objective according to claim 4, wherein theprojection objective comprises at a first mirror, a second mirror, athird mirror, a fourth mirror a fifth mirror and a sixth mirror, andwherein the first, second, third and fourth mirrors.
 7. The projectionobjective according to claim 6, wherein the second sub-objectivecomprises the fifth mirror and the sixth mirror.
 8. The projectionobjective according to claim 1, wherein the projection objectivecomprises at least six mirrors.
 9. The projection objective according toclaim 1, wherein the image-side numerical aperture is equal to or largerthan 0.2.
 10. The projection objective according to claim 1, wherein theprojection objective comprises at a first mirror, a second mirror, athird mirror, a fourth mirror, a fifth mirror and a sixth mirror, andwherein the first mirror is convex-shaped, the second mirror isconcave-shaped, the third mirror is concave-shaped, the fourth mirror isconvex-shaped, the fifth mirror is convex-shaped, and the sixth mirroris concave-shaped.
 11. The projection objective according to claim 1,wherein an intermediate image is formed in the light path from theobject plane to the image plane.
 12. The projection objective accordingto claim 11, wherein the projection objective comprises at least a firstmirror, a second mirror, a third mirror, a fourth mirror, a fifth mirrorand a sixth mirror, and wherein the intermediate image is formed betweenthe fourth mirror and the fifth mirror.
 13. The projection objectiveaccording to claim 12, wherein the first mirror is concave-shaped, thesecond mirror is concave-shaped, the third mirror is convex-shaped, thefourth mirror is concave-shaped, the fifth mirror is convex-shaped, andthe sixth mirror is concave-shaped.
 14. The projection objectiveaccording to claim 12, wherein an aperture stop is arranged a locationselected from the group consisting of between the second mirror and thethird mirror, and on the third mirror.
 15. The projection objectiveaccording to claim 11, wherein the image-side numerical aperture NA isequal to or larger than 0.2.
 16. The projection objective according toclaim 1, wherein the projection objective is a catoptric projectionobjective.
 17. The projection objective according to claim 1, whereinthe projection objective is a catadioptric projection objective.
 18. Amicrolithography projection exposure apparatus, comprising: anillumination system, a projection objective, wherein the projectionobjective has a negative back focus of the entry pupil and themicrolithography projection exposure apparatus has a projectionobjective according to claim
 1. 19. The microlithography projectionexposure apparatus according to claim 18, wherein by mirroring of theentry pupil at the object plane a mirrored entry pupil of the projectionobjective is formed to the image side of the object plane, and whereinthe illumination system includes an optical integrator that is formed inor near a plane in which the mirrored entry pupil is formed.
 20. Themicrolithography projection exposure apparatus according to claim 19,wherein the optical integrator is a facetted mirror.
 21. Themicrolithography projection exposure apparatus according to claim 19,wherein the optical integrator is a diffuser.
 22. The microlithographyprojection exposure apparatus according to claim 21, wherein thediffuser comprises more than 500 mirror facets.
 23. The microlithographyprojection exposure apparatus according to claim 21, wherein thediffuser is a holographic lattice grid.
 24. The microlithographyprojection exposure apparatus according to claim 21, wherein an aperturestop is arranged in the light path before the diffuser.
 25. Themicrolithography projection exposure apparatus according to claim 21,wherein at least one mirror is arranged in the light path after theoptical integrator and before the object plane.
 26. The microlithographyprojection exposure apparatus according to claim 21, wherein in thelight path from the light source to the diffuser a grazing-incidencemirror is arranged before the diffuser.
 27. The microlithographyprojection exposure apparatus according to claim 26, wherein thegrazing-incidence mirror has a mirror surface that includes anextra-axial conical surface.
 28. The microlithography projectionexposure apparatus according to claim 26, wherein the grazing-incidencemirror or the normal-incidence mirror comprises a spherical, toroidal,or aspherical part.
 29. The microlithography projection exposureapparatus according to claim 18, wherein the illumination systemcomprises a first facetted optical element with a plurality of firstraster elements as well as a second facetted optical element with aplurality of second raster elements, wherein the second facetted opticalelement is arranged in or near the plane of the mirrored entry pupil.30. The microlithography projection exposure apparatus according toclaim 29, wherein a first raster element has substantially the shape ofa field that is formed in the object plane.
 31. The microlithographyprojection exposure apparatus according to claim 30, wherein a firstraster element is configured with an arcuate shape.
 32. Themicrolithography projection exposure apparatus according to claim 18,wherein by mirroring of the entry pupil at the object plane a mirroredentry pupil is formed in a mirrored entry pupil plane to the image sideof the object plane, and wherein the illumination system includes anoptical integrator, and wherein the optical integrator is not arrangedin or near the mirrored entry pupil plane.
 33. The microlithographyprojection exposure apparatus according to claim 32, wherein the opticalintegrator is not arranged in a plane that is conjugate to the objectplane.
 34. The microlithography projection exposure apparatus accordingto claim 32, wherein the optical integrator has more than 1000individual facets.
 35. The microlithography projection exposureapparatus according to claim 32, wherein the optical integratorilluminates a field in the object plane, wherein the field has a fieldshape.
 36. The microlithography projection exposure apparatus accordingto claim 35, wherein the optical integrator has a shape, and whereinsaid shape is substantially the same as the field shape.
 37. Themicrolithography projection exposure apparatus according to claim 32,wherein the optical integrator is kidney-shaped.
 38. Themicrolithography projection exposure apparatus according to claim 32,wherein the optical integrator has a plurality of facets, and whereineach facet illuminates a plurality of discrete field points in theobject plane and of pupil locations in a pupil plane.
 39. Themicrolithography projection exposure apparatus according to claim 32,wherein a normal-incidence mirror is arranged in the light path from thelight source to the optical integrator.
 40. The microlithographyprojection exposure apparatus according to claim 39, wherein thenormal-incidence mirror has a mirror surface which includes anextra-axial conical surface.
 41. The microlithography projectionexposure apparatus according to claim 39, wherein the normal-incidencemirror has a mirror surface that is configured as a free-form surface.42. The microlithography projection exposure apparatus according toclaim 41, wherein the free-form surface is of a kind that has anextra-axial conical component.
 43. The microlithography projectionexposure apparatus according to claim 32, wherein in the light path fromthe light source to the object plane a normal-incidence mirror isarranged after the optical integrator.
 44. A microlithography projectionexposure apparatus, comprising: an illumination system that is traversedby an illumination ray bundle: an object plane in which an object fieldis being illuminated; a projection objective having a non-obscured exitpupil, wherein the projection objective is traversed by animage-projecting light path from the object plane to an image plane, andthe projection objective has a negative back focus of an entry pupil,and the microlithography projection exposure apparatus is configured insuch a way that the illumination ray bundle which travels from anext-to-last optical component in the light path of the illuminationsystem to the object plane does not cross over the image-projectinglight path except for the area where a reflection on the object planetakes place.
 45. The microlithography projection exposure apparatusaccording to claim 44, wherein the illumination ray bundle of theillumination system and the image-projecting ray path of the projectionobjective do not crossing over each other in the meridional plane in alight path from the object plane to the image plane.